The present invention relates in general to a current spreading circuit which in a preferred version is embodied in a DC power supply circuit characterized by improved (higher) power factor and decreased current crest factor. More particularly, the present invention relates to a pulse width modulation (PWM) current spreading circuit, which in one embodiment is implemented by the addition of a relatively simple control circuit to the standard rectifier and storage capacitor power supply circuit.
Steady-state operation of AC operated power supplies having the usual rectifier-capacitor input circuit, is characterized by an undesirable current peak which occurs at the maximum amplitude excursion of the sine voltage waveform. This peak current is due to energy transfer occurring only when the input sine wave voltage is higher than the sum of the rectifier drop and the capacitor voltage. Refer to FIG. 5 which will be described in further detail hereinafter. The time of energy transfer is typically 40.degree. to 60.degree. out of 180.degree. resulting in a high peak current as noted. Such peak currents are of concern, not only to power supply designers and users, but also to the power generation and transmission operators and customers sharing the same distribution source of electrical power.
The effect of these peak currents provides for undesired low power factor and increased current crest factor causing excessive current to be drawn for a given watt level, with VA values being substantially larger than watt values. In this connection, the current crest factor is defined as the ratio of the peak current to the RMS value of the current. Crest factors may be in the range of 3 to 4 and run as high as 10 and possibly even higher.
Accordingly, it is an object of the present invention to provide a circuit for providing current spreading to provide improvement in power factor. In its simplest terms, there is a mitigation of the current crest so as to redistribute the rectified AC input current drawn over a greater portion of the 180.degree. half wave. Ideally, the power supply and its load is to present an I.sub.in =sin .theta. view to the source, if it is to be a resistance equivalent. The redistribution of the rectified AC input current drawn generally requires the transfer of energy to the DC output bus of the power supply and takes place when the instantaneous voltage of the input AC sine wave is lower than the voltage on the DC output bus.
Another object of the present invention is to provide a pulse-width-modulation (PWM) current spreading circuit in accordance with the preceding object and which is relatively simple in design and which can be implemented without any substantial increase in cost as far as the overall cost of the power supply circuit is concerned.